1. Field of the Invention
The present invention relates to a photocoupler driving system and a camera using the system.
2. Description of the Related Art
Recent advances of semiconductor manufacturing processes have brought minuter elements for a CPU or the like to operate at high speed and with low power consumption. However, this has reduced an operating power supply voltage range and resistance to voltage of the element.
This tendency has been also found in CPUs used in cameras, and power consumption has been simultaneously reduced. In an electric circuit of the camera, an AF sensor which processes minute signals, or a flash control circuit which controls a high voltage power supply requires a high voltage (generally 5V), while a CPU which controls them, especially, a CPU using a high speed CPU core uses a low voltage core with minuter patterns to meet the operating speed. In this case, the CPU operates at a low power supply voltage different from a power supply voltage of the peripheral sensor or the like, and for such an electric circuit of the camera, it is necessary to prepare power supplies with different voltages and feed an optimal voltage from each power supply to the peripheral sensor or the like.
On the other hand, the camera uses a photocoupler such as a photointerrupter (PI) or a photoreflector (PR) for detecting opening of a shutter member, a position of a film in film feeding, a position of a photography lens barrel, or the like.
The photointerrupter includes an LED for light emission and a phototransistor for receiving a light emitted from the LED, and has a movable member which transmits and intercepts the light from the LED between the LED and the phototransistor. The photointerrupter detects whether the movable member intercepts the light, that is, the movable member is in a slit (between the LED and the phototransistor) of the photointerrupter, by energizing the LED on the light emission side and detecting the light emission of the LED with the phototransistor on the light receiving side.
The photoreflector detects whether the movable member is within a detecting range of the photoreflector, that is, whether the movable member is in a position where it reflects the light emission from the LED, by reflecting the light emission from the LED on the movable member and detecting the reflected light with the phototransistor on the light receiving side.
FIG. 7 shows an electric circuit of a camera including a photocoupler such as a photointerrupter or a photoreflector.
To an LED 105a of a photocoupler 105, limited resistance 106 and a transistor for switching (switching transistor) 107 connect in series, and when the switching transistor 107 is switched on by an instruction from a CPU 101, a substantially constant current feeds into the LED 105a. Therefore, the LED 105a emits a light and the phototransistor 105b receives the light from the LED 105a. 
On the other hand, detecting resistance 108 is incorporated between the phototransistor 105b and a GND, and when the phototransistor 105b receives the light from the LED 105a, a voltage in accordance with photocurrent from the phototransistor 105b is generated in the detecting resistance 108. The generated voltage is A/D converted by an A/D converter 102 included in the CPU 101 and then detected as a digital value. The CPU 101 controls desired operations, for example, opening a shutter member or film feeding based on the detected results. The voltage generated in the detecting resistance 108 can be detected by a comparator or the like as well as the A/D converter.
When the phototransistor 105b receives no light from the LED 105a, no current passes through the phototransistor 105b, and the voltage generated in the detecting resistance 108 becomes zero.
On the other hand, when the phototransistor 105b receives the light from the LED 105a, the current starts passing through the phototransistor 105b, and as the current increases, the voltage generated in the detecting resistance 108 increases. When the current passing through the phototransistor 105b further increases, the voltage generated in the detecting resistance 108 approaches the power supply voltage, and the voltage generated in the detecting resistance 108 increases up to a saturation level of the phototransistor 105b. 
The LED 105a requires this forward voltage (Vf) of 1 to 2 V, and a voltage of 1 V for controlling energizing of the LED 105a on/off and driving a substantially constant current, so that the circuit including the LED 105a requires a total voltage of 2 to 3 V. On the other hand, an operation of the light receiving side (phototransistor 105b) is allowed with caution not to saturate the phototransistor 105b. 
Generally, when the photocoupler 105 is driven, a stabilized voltage is used as a power supply of the LED 105a or the phototransistor 105b instead of a direct battery voltage so as to prevent influence of fluctuations in power supply voltages due to changes in current consumption in driving the movable member (such as the shutter). Specifically, the battery voltage is increased and stabilized by a DC/DC converter, and the output of the DC/DC converter to be used as a power supply of an AF sensor or the like is used as a power supply of the LED 105a or the phototransistor 105b. The increased and stabilized voltage is generally set to 5 V.
However, if semiconductor devices such as a CPU have become minuter to reduce resistance to voltage thereof, semiconductor devices including the detecting resistance 108 for detecting output of the phototransistor 105b or a detecting circuit such as the A/D converter 102 (or a comparator) have also become minuter to reduce the operating power supply voltage and the resistance to voltage thereof, preventing the conventional power supply voltage of 5 V from being applied.
Thus, the power supply voltage of the semiconductor device including the detecting circuit has to be set to a low voltage value such as 3.3 V or 2.5 V, or further, 1.8 V. In this case, if the circuit is used where the power supply voltage on the light receiving side (phototransistor 105b) is set to a 5 V system as is conventional, no problem occurs when amount of received light of the phototransistor 105b is small, but the semiconductor device cannot function normally when the amount of received light increases and, for example, when the voltage generated in the detecting resistance 108 exceeds the power supply voltage of the semiconductor device.
To solve this problem, it is possible to take measures in respect of the circuit or the process such as building a limiter of the power supply voltage into the semiconductor device or increasing the resistance to voltage only in the detecting circuit, but this raises costs significantly and is difficult to achieve.
If the power supply voltage identical to that of the semiconductor device (low value power supply voltage) is used as the power supply for emitting and receiving light in the photointerrupter or the photoreflector, an output voltage in the circuit on the light receiving side does not exceed the resistance to voltage of the semiconductor device, and the above described problem of the semiconductor device not functioning normally does not occur, but it becomes difficult to ensure the voltage for driving the above described LED, disabling desired light emitting control.
The present invention has an object to provide a camera ensuring stable operations of a light emitting element forming a photocoupler and a processing circuit in which elements becomes minuter to reduce resistance to voltage.
In order to attain the above described object, a camera according to the invention includes:
a first power supply output circuit which outputs a first stabilized power supply voltage;
a second power supply output circuit which outputs a second stabilized power supply voltage lower than the first power supply voltage;
a photocoupler which has a light emitting element and a light receiving element;
a detecting circuit which detects an analog signal output from the light receiving element and converts the analog signal to a digital signal; and
a processing circuit which performs processing based on the digital signal.
The first power supply voltage feeds into the light emitting element as an operating voltage of the light emitting element, the second power supply voltage feeds into the light receiving element as an operating voltage of the light receiving element, and the second power supply voltage feeds into the processing circuit as an operating voltage of the processing circuit.
A camera according to the invention also includes:
a first power supply output circuit which outputs a first power supply voltage;
a second power supply output circuit which outputs a second power supply voltage lower than the first power supply voltage;
a third power supply output circuit which outputs a third power supply voltage lower than the first power supply voltage;
a photocoupler which has a light emitting element and a light receiving element;
a detecting circuit which detects an analog signal output from the light receiving element and converts the analog signal to a digital signal; and
a processing circuit which includes at least a part of the detecting circuit and performs processing based on the digital signal.
The first power supply voltage feeds into the light emitting element as an operating voltage of the light emitting element, the second power supply voltage feeds into the processing circuit as an operating voltage of the processing circuit, the third power supply voltage feeds into the light receiving element as an operating voltage of the light receiving element, and the third power supply voltage feeds into the detecting circuit as an operating voltage of the detecting circuit.
In the above described invention, the light emitting element and the light receiving element formed as one unit such as a photointerrupter or a photoreflector can be used as the photocoupler.
When the camera includes a plurality of photocouplers, the light emitting elements in the plurality of photocouplers may be connected in series to feed the first power supply voltage into the plurality of light emitting elements, and feed the third power supply voltage into each of the light receiving elements in the plurality of photocouplers.
In order to attain the above described objects, a circuit for a photocoupler of the invention includes:
a photocoupler which includes a light emitting element and a light receiving element;
a power supply circuit which feeds a first driving voltage into the light emitting element and feeds a second driving voltage lower than the first driving voltage into the light receiving element;
an impedance element which is connected to the light receiving element, a current in accordance with an output current of the light receiving element passes through the impedance element; and a processing circuit.
An output voltage of the impedance element being input to an input end of the processing circuit, and the processing circuit being driven by a voltage lower than the first driving voltage.
A circuit for a photocoupler of the invention also includes:
a photocoupler which includes a light emitting element and a light receiving element;
a power supply circuit which feeds a first driving voltage into the light emitting element and feeds a second driving voltage lower than the first driving voltage into the light receiving element; and a processing circuit.
Output of the light receiving element being input to the processing circuit to process the output of the light receiving element, the processing circuit being driven by a voltage lower than the first driving voltage.
A detailed configuration of the camera of the invention, the above and other objects and features of the invention will be apparent from the embodiments, described below.